Polymeric electrical insulation for electrical conductors, e.g. as coatings for aerial drop wire, appliance wire, hookup wire and ignition wire requires, in addition to excellent electrical properties such as low dielectric constant and dissipation factor, the ability to be processed at high rates of shear and at high speeds into electrically insulating coatings having smooth surfaces and high stress crack resistance. Prior used nonchlorinated or highly chlorinated polyethylenes, i.e., polyethylenes containing at least about 30 percent by weight chlorine as well as polyethylenes containing minor amounts of a modifying polymer such as polypropylene (as described in U.S. Pat. No. 3,355,520); are difficulty processable and are characterized by relatively high power requirements, even at low line speeds, when being formed into electrical insulation for wire. Dry blending of additives such as stabilizers, blowing agents and fillers and the like has also proven to be ineffective when using prior used olefin polymers for extrusion coated electrical wire insulation having adequate surface smoothness and stress crack resistance.
Further, electrical insulation requires high resistance to thermal and/or oxidative induced degradation. In this regard, a material such as polyvinyl chloride (PVC), when irradiated with sufficient energy to provide thermal resistance, exhibits excessive degradation which makes it brittle and highly susceptible to oxidative degradation. Materials such as neoprene and chlorosulfonated polyethylene, when cross-linked, also become deficient in thermal stability and lose their flexibility upon aging. The problem of degradation of PVC may be overcome to some degree by the addition of added sensitizing monomer which enhances the cross-linking efficiency of such material. However, since such monomers are expensive and must be uniformly incorporated into the PVC, economics of the insulation are affected adversely. For olefin polymers cross-linking occurs due to abstraction of hydrogen atoms from molecules in the polymer chain. This abstraction of hydrogen form sites which join together instantaneously to form permanent bonds between adjacent polymer chains. In general, the irradiation efficiency of polyethylenes needs improvement and added sensitizing monomers have been used for this purpose with accompanying economic disadvantages.
It is a primary object of the present invention to provide electrical insulation formed from certain lightly chlorinated, high bulk density, linear olefin polymers which olefin polymers are characterized by improved processability and which may thus be processed at high shear rates and at high speeds into electrical insulation for wire and the like, such insulation having smooth surfaces and high stress crack resistance.
It is a further object to provide such electrical insulation wherein necessary additives such as stabilizers, blowing agents and fillers may be advantageously dry blended with said olefin polymer prior to processing of such polymer into electrical insulation.
It is another object to provide electrical insulation having excellent electrical properties and, in addition, enhanced response to cross-linking by exposure to atomic radiation, even in the absence of added sensitizing monomers, and which are characterized by exceptionally high resistance to thermal and/or oxidative induced degradation.